An EVT utilizes differential gearing and one or two electric motor/generators, also referred to as “motors” for simplicity, to allow adjustment of a speed ratio of a transmission input over a transmission output. An EVT may be particularly useful in a hybrid electric vehicle when used in conjunction with an electric energy storage device, such as a battery. An EVT may be able to save fuel relative to certain conventional automatic transmissions, in part because the EVT uses the energy-conserving action of the electric motors rather than the energy-dissipating action of a hydraulic torque converter. Additionally, an EVT provides the ability to continuously vary a transmission ratio, as well as the ability to recover energy during a braking event or slowing of the vehicle and to store this captured energy within the energy storage device. Finally, an EVT can assist a smaller and generally more efficient engine in propelling the vehicle.
An EVT may have a single range, also referred to as a “mode”, wherein the input speed, the output speed, and one or more motor speeds are a linear combination of one another. For example, the output speed may be one third of the sum of twice the speed of the input and the speed of one electric motor. Alternately, an EVT may have multiple ranges/modes to reduce the motor power required for adjusting the transmission speed ratio and a transmission or transfer of power from the input to the output.
The multiple ranges/modes described above are activated by the engagement of selectable torque-transmitting devices or clutches. For example, a two-mode EVT may have one range for high transmission speed ratios, which is activated by the engagement of one clutch, and another range for low transmission speed ratios, which is activated by engagement of another clutch. Since each of these ranges is continuously variable, the EVT may be designed so that shifting between the two ranges may be accomplished with the transmission operating at a particular speed ratio which is common to both ranges. Therefore, the shift event may be made synchronously, that is, with zero relative speed across the clutches during the shift event. The shift event is then simply a torque transfer between one clutch and another clutch without the necessity of slipping between the clutches, while the electric motors control the speed ratio through the transmission both before and after the shift event.